How to select elements to minimize the objective function.

Suppose a set has 4 elements, ;For another set ,where n=1,2,...,64, and bis random number in [0.45,0.55]. Select a element from for each n, and get a select matrix (64×4), where represents the nth seletion is ith element in . So only one element in each column of matrix is 1, others are 0.

The objective function is , which means how to select element in in each n to get objectve function minimum.